NOR#

NOR is OR followed by inversion: the output is high only when all inputs are low, and low if any input is high. Like NAND, it is a universal gate — and it holds a special place in the history of the field.

Universal, and the Mirror of NAND#

NOR is functionally complete: an inverter is a NOR with its inputs tied together, and every other gate follows. It is the De Morgan dual of NAND — a NOR behaves as an AND with inverted inputs — so a design can be built entirely from NOR just as it can be built entirely from NAND. In CMOS it is equally native and cheap: four transistors for a two-input gate, where a true OR needs six.

A Whole Computer From One Gate#

The clearest demonstration of universality is historical: the Apollo Guidance Computer was built almost entirely from a single part — a three-input NOR gate — around 2,800 of them. One gate type, chosen for exactly the completeness described above, was enough to implement an entire flight computer. It is the sharpest illustration of what “functionally complete” really means, and a recurring theme of this book that integration simply packed more of such gates onto less silicon.

Beyond Combinational#

Cross-couple two NOR gates and the result is an SR latch with active-high set and reset — the complement of the NAND latch, and the more intuitive of the two to read, since a high on Set sets and a high on Reset resets. Memory, like every gate, falls out of NOR alone.